1. Field of the Invention
This invention relates to manganese-zinc base ferrite, and more particularly, to high permeability manganese-zinc base ferrite suitable as cores in broad-band transformers.
2. Description of the Background
For broad-band transformers such as pulse transformers, a core-forming manganese-zinc base ferrite having a high magnetic permeability over a broad band, and especially throughout the band of 10 to 500 kHz is needed in order to enable accurate digital communication.
The electromagnetic properties such as permeability and loss of manganese-zinc base ferrite have structure sensitivity and are largely affected by its micro-structure. In general, the permeability of such ferrite is increased by selecting a composition giving a low crystal magnetic anisotropy constant and a low magnetostriction constant, increasing the grain size, reducing vacancy, and increasing the sintered density. This is because the above choice facilitates magnetic wall migration. It is believed that permeability is governed by magnetic wall migration. Also auxiliary components have a substantial influence on the magnetic properties. Unless the contents of auxiliary components are controlled, the auxiliary components can prevent smooth migration of magnetic walls due to precipitation on grain boundaries, promoted abnormal grain growth and promoted void formation, leading to a decline of permeability.
JP-B 5-55463 discloses a sintering type oxide magnetic material comprising ferric oxide, manganese oxide and zinc oxide as main components, and up to 0.01 wt % of silicon dioxide and up to 0.02 wt % of calcium oxide as auxiliary components, characterized in that the material further contains 0.02 to 0.05 wt % of bismuth oxide and 0.005 to 0.05 wt % of aluminum oxide as auxiliary components, and has an initial permeability xcexc0 of at least 18,000 and a relative loss factor tanxcex4/xcexc of up to 2.0xc3x9710xe2x88x926; an oxide magnetic material in the form of a sintered body produced by sintering the above material at 1300 to 1370xc2x0 C.; and an oxide magnetic material in which the temperature coefficient of xcexc0 is always kept positive due to addition of aluminum oxide. Although the magnetic material described in this patent is allegedly suitable for use as communication transformer cores, it fails to provide the high permeability character required to be commensurate with higher transmission speeds.
As the technique relating to large grain sizes, JP-B 52-29439 discloses a method of preparing a polycrystalline manganese-zinc ferrite body by shaping a fine ferrite-forming mixture into an article of desired shape, followed by compression and sintering, characterized in that a grain growth promoting substance selected from among SrF2, oxides of B, Bi, Ca, Cu, Mg, Pb, Si and V, and Fe3(PO4)2 and mixtures of any is added in an amount of 0.005 to 1 wt % based on the ferrite at any preparation stage prior to the sintering step, and the sintering is conducted at a temperature of 1350 to 1400xc2x0 C. until conjugated crystals having a mean grain size of at least 50 xcexcm are formed. However, the application of this ferrite is limited to heads. Its object is to provide a head having improved anti-decay and long lifetime characteristics by virtue of irregularly conjugated crystals having a large grain size, unlike the object of the present invention to provide a high permeability by making use of very large crystal grains, and the high permeability characteristics required for pulse transformers in transmitters are not available. Additionally, this ferrite, intended to be applied to heads, does not have a high permeability as required in pulse transformers in transmitters.
Under the circumstances, the present applicant proposed in JP-A 6-204025 a manganese-zinc base ferrite having a high permeability over a broad band and especially throughout the band of 10 to 500 kHz. Disclosed in this patent is a manganese-zinc base ferrite comprising 50 to 56 mol % calculated as Fe2O3 of iron oxide, 22 to 39 mol % calculated as MnO of manganese oxide, and 8 to 25 mol % calculated as ZnO of zinc oxide, which is sintered after addition of up to 800 ppm calculated as Bi2O3 of bismuth oxide and up to 1200 ppm calculated as MoO3 of molybdenum oxide.
The manganese-zinc base ferrite disclosed in this patent exhibits a high initial permeability over a broad band as demonstrated by an initial permeability of at least 9000, at least 9000 and at least 3000 at 10 kHz, 100 kHz and 500 kHz and 25xc2x0 C., respectively.
In order to accomplish size reduction and higher transmission speed for pulse transformers, it is crucial for ferrite to have a higher permeability especially in a frequency region near 10 kHz. The provision of a high permeability permits a smaller number of windings to provide a higher inductance and a lower distributed capacity and enables passage of signals in a broad band.
Meanwhile, aiming to provide a manganese-zinc base ferrite having a high permeability over a broad band (typically a permeability of at least 9000 in a frequency region of 10 to 100 kHz) and especially, a high permeability throughout a frequency region of 10 to 500 kHz, JP-A 6-204025 of the present applicant proposes a manganese-zinc base ferrite comprising 50 to 56 mol % calculated as Fe2O3 of iron oxide, 22 to 39 mol % calculated as MnO of manganese oxide, and 8 to 25 mol % calculated as ZnO of zinc oxide, which is sintered after addition of up to 800 ppm calculated as Bi2O3 of bismuth oxide and up to 1200 ppm calculated as MoO3 of molybdenum oxide. This ferrite, however, fails to provide the necessary high permeability characteristics.
An object of the invention is to provide a manganese-zinc base ferrite having a high permeability over a broad band and especially, a high permeability in a frequency region in proximity to 10 to 100 kHz.
This and other objects are achieved by any of the following constructions (1) to (7).
(1) A manganese-zinc base ferrite comprising iron oxide, manganese oxide and zinc oxide as main components in amounts calculated as Fe2O3, MnO and ZnO, respectively,
and 0.0003 to 0.003% by weight calculated as P of phosphorus as an auxiliary component, and having a mean grain size from more than 50 xcexcm to 200 xcexcm.
(2) The manganese-zinc base ferrite of (1) further comprising more than 0% to 0.08% by weight of bismuth oxide as an auxiliary component, based on the main components and calculated as Bi2O3.
(3) The manganese-zinc base ferrite of (1) or (2) further comprising more than 0% to 0.12% by weight of molybdenum oxide as an auxiliary component, based on the main components and calculated as MoO3.
(4) The manganese-zinc base ferrite of any one of (1) to (3) further comprising at least one oxide of niobium oxide, tantalum oxide and zirconium oxide as an auxiliary component in amounts calculated as Nb2O5, Ta2O5 and ZrO2, respectively,
(5) The manganese-zinc base ferrite of any one of (1) to (4) further comprising 0.005 to 0.05% by weight, calculated as CaO, of calcium oxide.
(6) The manganese-zinc base ferrite of any one of (1) to (5), having a permeability of at least 15,000 at 10 kHz.
(7) The manganese-zinc base ferrite of any one of (1) to (6), having a permeability of at least 15,000 at 100 kHz.